1. Field of the Invention
The present invention relates to a defect compensation method for a semiconductor element to compensate for defects of the semiconductor element in the production of the semiconductor element, and particularly relates to the defect compensation method for the semiconductor element to compensate for defects of the semiconductor element in the production of a functional semiconductor element having a semiconductor junction, such as photovoltaic element and a thin film transistor.
2. Related Background Art
Conventionally, when the semiconductor element such as the thin film transistor (TFT) is produced, in the deposition of a semiconductor film, an insulating film, and a metal film, according to a difference in conditions (temperature of substrate, pressure, discharge power, or the like) of deposition of respective films, there are occasions in which many defects caused by strain occur inside respective films and at interfaces between respective films. Besides, there are occasions in which many defects, which are caused by the difference in thermal expansion coefficients and lattice constants of respective films, occur in respective films. Such defects of the thin film transistor function as an electric charge trap and form a space charge in the thin film transistor to thereby lower electric characteristics of the transistor and lower the reproducibility of electric characteristics.
In the case of a silicon-based, thin film, photovoltaic element, a reflecting film and reflection enhancing film, n-type semiconductor film, substantially i-type semiconductor film, p-type semiconductor film (nip structure comprising these films may be stacked a plurality of times as the occasion demands), transparent conductive film (or metal film), and current collecting electrode are configured on a support made of a metal and glass. Particularly, in the case where a microcrystalline semiconductor film and polycrystalline semiconductor film are used as i-type semiconductor film, the crystal grain diameter increases as a semiconductor crystal grows from the support side in the formation of the semiconductor film. As a result, in addition to the above-described defects occurring in the formation of the thin film transistor, a problem in which defects increase in crystal grain boundaries occurs.
In the case where the microcrystalline semiconductor film and polycrystalline semiconductor film are deposited by a plasma CVD method, hydrogen content in the film is decreased by an increase in the crystal grain diameter according to the growth of such films. As the result, defects in the crystal grain boundaries increase.
As the method for the defect compensation to reduce the above-described defects, conventionally proposed methods are atmospheric annealing in an oxygen atmosphere, reducing annealing in a hydrogen atmosphere, and hydrogen plasma annealing. However, these methods for defect compensation are low in the reactivities of oxygen molecules and hydrogen molecules. Therefore, even with a long period of atmospheric annealing, reduction of defects has been achieved insufficiently. In the case of using hydrogen plasma, hydrogen radicals become active and thus, hydrogen radicals etch an impurity in and around a chamber, in which hydrogen plasma is generated, to pose another problem to cause attaching of the impurity to the inside and surface of the semiconductor film.
As the method for solving these problems, the method using water vapor was proposed as described in Japanese Patent Application Laid-Open No. 8-55858. According to this method, in an atmosphere containing water gas of 20 to 400.degree. C. and a partial pressure of 1 Torr or higher and lower than the saturated vapor pressure, a heating step is effected for 15 seconds or longer and shorter than 20 hours. After passing through this heating step, quality of at least one of the semiconductor or the insulating film is changed.
However, the conventional method using water vapor as described in the above described Laid-Open gazette, easily causes a problem in the formation of an insulating film made of SiO, causing degradation of a hot carrier and causing water and OH.sup.- groups to remain in the film. When transistor characteristics superior to the present ones are required, water and OH.sup.- groups retained in the film should be further reduced.
By the method for using water vapor in microcrystalline silicon-based and polycrystalline silicon-based solar cells, oxidation of the crystal grain boundaries may excessively proceed to inhibit electric charge transfer. In the case of a microcrystalline silicon semiconductor deposited by the plasma CVD method, the microcrystalline silicon grows in a sector form from the substrate side toward the growing surface to cause strain inside the microcrystalline silicon semiconductor.
According to the conventional method using water vapor, the compensation is attempted for such defects of the crystal grain boundaries of the microcrystalline silicon semiconductor, and oxidation by water vapor may excessively proceed to expand the crystal grain boundary, increase the internal strain of the microcrystalline silicon semiconductor, and lower the conductivity of the microcrystalline silicon semiconductor.